The invention relates to an arrangement of self-purging centrifugal separators for the separation and clarification of liquids, especially heavy oils, which contain solids, at least two separators being connected in series and the first separator serving as a preliminary separator and the second as a clarifier, the purging of the solids from each separator being timed by means of an associated control apparatus.
Such arrangements of self-purging separators are known on board ships, for example, wherein the separation of the heavy oils from water and solids is performed in the first separator, and the clarification of the oil separated and partially purified in the first separator takes place in the second separator. In this separating and clarifying process, a larger amount of the solids contained in the untreated mixture is normally separated in the drum of the first separator than in the drum of the second, clarifying separator.
The self-purging separators used for this purpose have been disclosed, for example, by German Pat. No. 1,297,036, and they are especially suitable for the continuous and automatic operation that is required on board ships and also in numerous other separator installations. In such installations the solids that are separated in the drums are automatically discharged by the partial or complete purging, at certain intervals of time, of the separating chamber or solids chamber of the drum. The operations of purging the drums are initiated mostly by timing controllers such as those known from German Pat. No. 1,142,795.
Self-purging separators in which the purging of the drum is started by timing apparatus can be operated continuously only if the conditions of operation do not change, i.e., the solids content in the liquid and the rate of feed of the latter remain substantially constant (which is rarely the case with heavy oil), such that irregularities in these installations are unavoidable. In the performance of the so-called partial purges that are often practiced in these centrifuges, only a very specific portion of the drum content--normally that corresponding to the solids that have been separated--is to be ejected. If the solids content in the raw liquid is variable, however, while the timing of the purges is fixed, then, when the solids content diminishes, too much of the clear phase, e.g., oil, is ejected together with the solid, and, when the solids content increases, the purge is performed too late, so that too much of the solids can collect in the drum, thus making the removal of the solids difficult both in partial and in total purging operations. In addition, the separating insert built into the drum can become clogged with the solids, thereby greatly reducing the separating effect in the drum or even completely defeating it. Since these perturbances occur very frequently in separators in which heavy oil is processed, measures must be found for automatically eliminating trouble or at least sounding the alarm so that manual remedying of the trouble can be performed.
This is due to the fact that heavy oils, which are subject to very great variations in their content of solids and water, are of a dark and viscous consistency, so that the solids tend to cake up in the solids chamber, they are often very difficult to remove, and the quality of the oils cannot be determined at the outlet of the separator on account of their dark color. For this reason, it is impossible, for example, to monitor the oil outlet of the separator by means of a photoelectric cell, as can be done in the case of other liquids.